This invention relates to injection molding machines and in particular to an improvement in injection molding machines of the type having a nozzle mounted to inject a pressurized molten plastic into a mold cavity through a sprue and associated runner system. Typically, such injection molding machines employ a stationery platen mounted to one mold half with an opening into the platen at the sprue of the mold. The tip or outlet end portion of the nozzle is mounted against a sprue bushing such that the molten plastic will be injected into the sprue. Although the nozzle to sprue bushing interface in these machines is designed to be a sealed interface to prevent leakage of the molten plastic from between the nozzle and sprue bushing, it is a known reocurring problem that such leakage does take place with resultant problems including, among others, delivery of an inadequate supply of plastic into the mold, loss of proper pressure, material waste, damage to sensitive controls and parts of the machine due to for example the high temperatures of the leaking molten plastic material and consequential machine down time for repairs and high finished part scrap. Nozzle leaks, although principally occuring at the interface of the nozzle tip and sprue bushing, are known to occur at other locations associated with the nozzle such as for example at the nozzle attachment point to the machine's barrel which delivers the molten plastic to the nozzle. Depending on where the nozzle leak occurs and the rate of leakage, the leaking molten plastic can flow over different courses or paths. For example, in one type of leak, the molten plastic mushrooms or billows out from between the nozzle tip and sprue bushing interface due to a poor seal into the air space surrounding the nozzle in the platen opening. Other leaks are of a slow glittering type which actually clings to and creeps down the length of the nozzle eventually encrusting the nozzle or reaching the sensitive areas associated with the machine's operations.
It would therefore be highly desirable to provide for means to detect leaks associated with the nozzle of such machines so that corrective measures can be initiated before damage to expensive parts of the molding machine or excess scraped parts is encountered.